1. Field of the Invention
The present invention relates to a power amplifier system. More specifically, the invention relates to an HF (high frequency) power amplifier system usable with a mobile phone terminal and an HF power frequency power amplifier system usable with a mobile phone terminal, and a mobile phone using the HF power amplifier system.
2. Description of the Related Art
As a tendency of recent mobile phones, the feed-forward speed of transmit/receive data increases. For example, according to high speed down Link packet access (HSDPA), a dedicated packet control channel (DPCCH) is added for high feed-forward speeds of data (downlink: 14 Mbps; uplink: 64 Kbps). Accordingly, a peak factor (rate between a peak power and an average power) is increased by about 1.5 dB in comparison to a normal case.
According to the EvDO (Evolution Data Only) Rev. 0, a supplement channel is added to thereby increase the feed-forward speed up to 156 Kbps. In this case, the peak factor is increased by about 1 dB, similar to the above.
Schemes are continually under development to achieve further increased feed-forward speed, and undoubtedly, the peak factors of signals will be further increased in the future. A feature of such a signal of a high peak factor is a high peak power. As such, a power amplifier for use is desired to be capable of implementing a high level saturation output, high efficiency, and low distortion. As a candidate of such an amplifier, an output-combining power amplifier, such as a Doherty amplifier, is now drawing attention.
Under these circumstances, various types of known or existing techniques are related to the present invention, and some of such techniques which will be briefly described herebelow as related art examples.
A first related art example is a “digital cross-cancellation (or “DCC”) system,” such as described in Patent Publication 1 (Japanese Unexamined Patent Application Publication No. 2005-45767) will be described here. According to the DCC system, a digital signal processor (or “DSP”) is used to cancel signal distortion components in an output portion of a power amplifier. However, for use with a mobile phone, desired compactness cannot be achieved, and power consumption is significantly large because of the DSP being used.
A second related art example is a “multistage Doherty amplifier,” such as described in Patent Publication 2 (PCT Japanese Translation Patent Publication No. 2003-536313). According to the technique, a large number of power amplifiers are used for drive amplifiers, and signal distortion generated by the respective power amplifier is minimized, thereby to implement distortion reduction. However, problems arise in that it is difficult to set a minimum distortion point of the respective amplifier, further, a distortion component contained in a final output is cannot be reduced lower than a distortion component generated in the respective amplifier.
A third related art example is a “microwave Doherty amplifier,” such as described in Patent Publication 3 (Japanese Unexamined Patent Application Publication No. 1995-22952). According to the technique, a second harmonic short circuit is combined with a Doherty amplifier, thereby to implementing high efficiency and distortion reduction. However, a distortion reduction effect of the technique using the second harmonic short circuit is, at most, 3 dB, which is insufficient as described in Non-Patent Publication 1 (see (*1) below)
(*1): S. Kusunoki, et al., “Harmonic-Injected Power Amplifier with 2nd Harmonic Short Circuit for Cellular Phones”, IEICE Transaction on Electronics vol. E88-C, No. 4, April 2005, pp. 729-738